How come every one thinks Elsa and Jack Frost would be a good couple, they're not even from the same movie?

Given their similarities, I understand why, but I simply do not ship it because of their age difference. Elsa is twenty-one, and Jack is only fourteen. If Jack were able to age and reached the age of twenty when Elsa would be twenty-seven, that wouldn't be as bad, but my understanding is that Jack will always remain fourteen.

It's because they both have snow powers.
You can ship people from different franchises, usually people do that if two characters are similar to each other.

I personally HATE "Jelsa", its not happening. People think its good 'cause their share of cryokinetic powers.